cin a fraction of unknown size?

Question

How would one go about overloading the cin >> operator in c++ to input a fraction of unknown size/chars? Specifically, the person can enter 1/2 or 12/4 or 1/100 and it has to be able to accept any of them and input them into a numerator and denominator in a fraction object. Is there a simply command to determine where a character is in a string and then do something different with the characters before and after it?

And I can't have the person hit return/enter after inputting the numerator and denominator slash. Such as 4 (enter) / (enter) 15 (enter). They must be entered in one line and manipulated from there.

Note: I did already look at the similar questions on this site, but none seemed to address fractions which weren't composed of a single int numerator and denominator.

Answer 1

here is a very simple example of extraction using istringstream

note that, you must not put spaces when reading; eg : 123 / 34 if you want that instead, just use : std::cin >> numerator >> dummy_char >> denominator;

#include <iostream>
#include <sstream>
#include <string>

int main()
{
    std::string str( "123/455" );
    std::istringstream is( str );
    std::size_t i, j = 0;

    if( !(is >> i) ) return 1; // extract

    is.get(); // get the '/'

    if( !(is >> j) ) return 1; // extract

    std::cout << i << '/' << j;
}

Answer 2

Just use cin to read an int, the slash, and another int.

int num, den;
string slash;

cin >> num >> slash >> den;

// optionally check that slash contains a slash if you don't trust the user
// if("/" != slash) abort();

EDIT: I just tried this, and it only works if there is a space after the slash before the denominator. If there are no spaces, you can change slash to a char.

EDIT2: If you had a Fraction struct such as the following,

struct Fraction
{
    int num, den;
};

then you can overload operator >> to read in the fraction directly.

istream& operator>>(istream &in, Fraction &f)
{
    string slash;
    return in >> f.num >> slash >> f.den;
}

// in main or somewhere
Fraction f;
cin >> f;

As before, you can change slash to char if there will be no spaces, and you may want to do some error checking before returning the in object from operator>>.

Answer 3

1. load the whole thing as a string,
2. use string.find() to find the /,
3. Split the string like noted in Split a string in C++?, and convert the parts to numbers using ether stringstream, atoi() or anything else,
4. ???
5. Profit.

(sorry for 4 and 5, couldn't resist myself)

Note: Of course you should do that inside an overloaded operator>> of your fraction class.

Answer 4

You want to read and match a / character between the numerator and denominator, so you need the missing operator>>(istream&, char) function:

std::istream &operator>>(std::istream &in, char ch) {
    while (std::isspace(in.peek())) in.get();
    if (in.peek() == ch)
        in.get();
    else
        in.setstate(std::ios_base::failbit);
    return in;
}

std::istream &operator>>(std::istream &in, fraction &fr) {
    in >> fr.num >> '/' >> fr.denom;
    return in;
}

That function (or something like it) really should be part of the standard library, but its easy enough to add it yourself.